1. Field of the Invention
This invention relates to woven papermakers fabrics, and more particularly to an improved woven papermakers fabric that exhibits substantially less machine direction stretch and shrinkage than other available fabric structures.
2. Description of the Prior Art
In papermaking machines, a papermakers belt in the form of an endless, belt-like fabric structure is supported on and advanced by a plurality of metallic rolls rotatably supported in the papermaking machine. The belt serves to transport paper during the various stages of its processing during the papermaking process, as it passes through the papermaking machine. Papermakers belts have various names, depending upon the portion of the machine in which they are used. By way of example, papermakers belts can include so-called forminq fabrics, wet press felts, and dryer felts and fabrics. In many cases, the belt or fabric is joined at its ends to form an endless belt that is supported on and controlled by various machine rolls forming part of the papermaking machine.
A papermakers fabric can be made from a one, two, three, or more plane fabric, wherein the various planes are defined by different groups of cross-machine direction yarns. The planes, plies, or layers, as they are variously called, are united by a plurality of machine direction yarns that are interwoven with the cross-machine direction yarns to form a coherent fabric that has desired surface, stability and permeability characteristics, depending upon the portion of the papermaking process in which it is used. In that regard, the yarns that are used to weave the most modern papermakers fabrics are often made from synthetic monofilaments, or synthetic mutilfilaments, and from such materials as polyester or polyamide.
By virtue of the interwoven structure of the typical papermakers fabric, both the cross-machine direction yarns and the machine direction yarns are crimped, or bent, as they pass above or below the respective yarns with which they are interwoven. Although after weaving the fabric is subjected to heat and tension to set the yarns in the desired relative orientation, regardless of the tightness of the weave, any crimped machine direction yarn will increase in length as the fabric is placed under tension. Such a result is undesirable in that it causes the fabric to stretch and lengthen in the machine direction. As the fabric tension must be kept constant during the paper making process, fabric stretch can cause the fabric to lengthen beyond the take-up capabilities of the paper machine in which case tension is lost and the fabric has to be removed, because it is too long. Furthermore, if the tension applied to the fabric is relatively low, the fabric may shrink back, again beyond the adjustment capabilities of the machine. In this case the tension builds up and can result in damage to the paper machine. Further complicating the situation is that the tension in the fabric as it runs is not constant and uniform along the fabric, which brings about fluctuations as the fabric travels through the papermaking machine.
One attempt to overcome stretching of a papermakers fabric on the papermaking machine involved the technique of overstretching the fabric by the fabric manufacturer during the finishing operation. However, it has been found that finishing a woven fabric by using high stretch forces will result in built-in high shrinkage and consequent shrink forces that cause the fabric to contract on the paper machine to the point of tension build-up and subsequent machine damage. On the other hand, finishing a fabric by using low stretch forces will reduce the likelihood of it contracting on low tension positions of the machine, but it will increase the likelihood of it stretching in high tension positions on the machine. However, because in many cases neither the papermakers fabric manufacturer nor the paper mill that uses the fabric knows precisely under what tension the fabric will be operating, there is always the possibility of either fabric stretching or fabric shrinking.
It is an object of the present invention to overcome the above-described problems associated with the prior art fabric structures, and to provide a papermakers fabric that has improved resistance to stretching when under tension after being finished using low stretch forces, such that it performs well on a large variety of papermaking machines under a variety of operating conditions.